Quantification of gene effects on single milk proteins in selected groups of dairy cows

Different alleles and genotypes per polymorphic milk protein were compared, within the concentration and quantify of the protein which is coded from the corresponding locus. Cows were chosen according to their diversity in milk-protein content, as well as in the polymorphic milk proteins. Polymorphic milk proteins were analysed by isoelectric focusing. The contents and quantities of total milk protein and whey protein were measured photometrically. Single milk proteins were quantified after polyacrylamide nel electrophoresis, using computer-assisted densitometer. The contents of single milk proteins could be measured with repeatabilities between 0.58 (alpha-lactalbumin) and 0.97 (beta-lactoglobulin). Close associations were observed between alleles/genotypes of milk-protein-coding gene loci, and the contents, as well as the yields, of the corresponding milk proteins. Superior alleles concerned with the content yield of corresponding proteins have been beta-lactoglobulin A, alpha(s1)-casein C, and beta-casein B. Simultaneous comparison of genotypes of the casein gene cluster with the milk-protein values revealed significant effects through distinct allele combinations. Breed influences on the associations between genes and milk proteins were considerably smaller than the influences from single milk-protein-coding loci. Intragenic haplotypes may explain the associations between variants in coding DNA sequences and gene expression, estimated for single milk proteins. An analysis of balancing forces on allele frequencies and haplotype combinations per population will be necessary, before genotype screening can be applied usefully for breeding.